A thermally sensitive recording medium having a thermally sensitive recording layer (called a thermally sensitive color-developing layer or a thermally sensitive layer) containing a colorless or pale colored dye precursor and a color-developing agent which develops color by a thermal reaction with the colorless or pale colored dye precursor as main components was disclosed in Japanese Patent S45-14039 B publication and is widely utilized. A thermal printer in which a thermal head is built in is used to record images on the thermally sensitive recording medium, and when compared with the conventional recording method, this thermally sensitive recording method has advantages that it is noiseless at the recording process, developing and fixing processes are not necessary, it is maintenance-free, an apparatus is relatively cheap and compact, and an obtained color is very clear. Therefore, it is widely applied as recording papers for industrial information such as a facsimile, a terminal printer of a computer, a recorder for a measuring instrument or a label. Recently, the uses are becoming diversified and, along with the diversification of uses, recording instruments are becoming compact and high speed. Accordingly, a thermally sensitive recording medium on which a clear developed image can be obtained by a small amount of thermal energy is desired.
For the purpose of satisfying these requirements, a method of enhancing the color-developing sensitivity by adding a thermal fusible substance in a thermally sensitive recording layer (Patent Document 1), a method of enhancing the color-developing sensitivity by using a novel color-developing agent having a high color-developing ability and a method of combining a specific color-developing agent and specific sensitizer (Patent Document 2, Patent Document 3 and Patent document 4) are disclosed. However, problems such as the deterioration of the heat-resistance of ground color, powdering by time lapse, deterioration of re-printing ability, deterioration of debris-adhering resistance or deterioration of sticking resistance. In particular, the deterioration of the debris-adhering resistance and deterioration of sticking resistance are becoming big problems. The deterioration of the debris-adhering resistance and deterioration of sticking resistance are caused by the fusing and adhering of components contained in a thermally sensitive color-developing layer by heat from a thermal head. Aiming to solve said problems, a method of containing fine particles of an amorphous silica having a specific particle size distribution, specific BET surface area and bulk density (Patent Document 5) is disclosed, however, because the surface activity of the silica promotes a reaction between a leuco dye and a color-developing agent, the problem of a ground color developing (background coloring) arises. Further, in the case when ordinary silica is used, since the surface strength (coating layer strength) of a thermally sensitive recording medium deteriorates, not only a problem of staining of a blanket arises at an offset printing, but also the head abrasion-resistance is deteriorated.
Patent Document 1 JP S56-169087 A publication
Patent Document 2 JP S56-144193 A publication
Patent Document 3 JP S60-82382 A publication
Patent Document 4 JP S57-201691 A publication
Patent Document 5 JP S58-87094 A publication